While Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, single-site (metallocene and non-metallocene) catalysts represent the industry's future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they often produce polymers with improved physical properties.
Since the mid-1980s, scientists have become increasingly interested in bimetallic metallocenes, and in particular, how two metal centers communicate with each other via electronic and through-space interactions (see, e.g., Reddy et al. Organometallics 8 (1989) 2107). Cooperative effects are most likely when the two metal centers are electronically coupled through a conjugated pi-electron system. Ultimately, understanding cooperative effects should let polyolefin manufacturers fine-tune polymer properties by varying catalyst structure.
Jungling et al. (J. Organometal. Chem. 460 (1993) 191) describes bimetallic complexes in which two zirconocenes are linked through the Cp rings via a 1,4-phenylene group. Soga et al. (J. Mol. Catal. A 128 (1998) 273) describes dinuclear metallocenes linked by a biphenyl group. More recently, others have studied the impact of changing the nature and length of the linking group. Noh et al. (J. Organometal. Chem. 580 (1999) teaches polymethylene- and polysiloxane-linked dinuclear metallocenes. For these bimetallic complexes, catalyst activity increases as the linking group becomes longer and the catalytic centers can behave more independently. Tian et al. (Macromol. Chem. Phys. 203 (2002) 31) synthesized a series of sila-linked dinuclear zirconocene complexes and also concluded that catalyst activity and polymer molecular weight are influenced by changing the nature and length of the linking group.
Organometallic complexes that incorporate “indenoindolyl” ligands are known (see U.S. Pat. Nos. 6,232,260 and 6,451,724). In many of the known complexes, an indenoindolyl group is bridged to another group, which may be a second indenoindolyl group. The ligands are versatile because a wide variety of indanone and arylhydrazine precursors can be used to produce indenoindoles. Thus, substituent effects can be exploited and catalyst structure can be altered to produce polyolefins having a desirable balance of physical and mechanical properties. The complexes disclosed in the '260 and '724 patents are monometallic.
One drawback of at least some of the indenoindolyl complexes is their relatively limited ability to produce polyolefins having a desirably low melt index. For example, our gas-phase ethylene polymerizations with an indenoindolyl(cyclopentadienyl)zirconium dichloride complex, performed in the absence of hydrogen, often failed to give linear low density polyethylene (LLDPE) having a melt index less than about 1. Ideally, a catalyst will give polymers with fractional melt indices (preferably 0.1-0.8) when the polymerization is performed in the absence of hydrogen.
We also found that conventional indenoindolyl complexes often provide limited opportunities for controlling molecular weight distribution (MWD). For example, the complexes typically give polyethylenes having melt index ratios (MIR) in a narrow window in the range of about 17-19, and this value is independent of the amount of comonomer or aluminum activator used. Because of their relatively low MIR values, the resulting polyolefins have a limited degree of processability. Ideally, the MIR value could be increased in a controlled way to enhance processability.
U.S. Pat. No. 6,414,162 describes bimetallic complexes that derive from dianionic indenoindolyl ligands. These complexes can include two metals, each of which is bonded to two indenoindolyl groups, and the indenoindolyl groups are not otherwise linked together.
In sum, there is considerable current interest in bimetallic complexes and their potential value as catalysts for the manufacture of polyolefins because bimetallic complexes can have electronically coupled active sites. Known indenoindolyl complexes, which are mostly monometallic, have limited ability to give polyolefins with desirably low melt indices and broadenable MWDs. The industry would benefit from the availability of new bimetallic catalysts, especially ones that can provide polymers with desirable attributes. Particularly valuable catalysts would capitalize on the inherent flexibility of the indenoindolyl framework.